Russia denies that its forces are backing rebels, who have been gaining ground on Ukrainian forces.

Mr Poroshenko said Ukraine was a victim of "military aggression and terror".

He said: "I think that we are very close to the point of no return. Point of no return is full-scale war.

"Any offensive action which would be undertaken [by Russia]... would be a point of no return. And that's why we undertake enormous efforts to stop that."

Mr Poroshenko said that new EU sanctions against Russia would be prepared and would be implemented depending on the success of a proposed peace plan.

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Media captionLithuanian President Dalia Grybauskaite says Russia is "practically in a war against Europe"

He said: "Based on my proposals, which were supported by the majority of the member states, there are possible sanctions that would be implemented."

He said he hoped to publish a draft of his peace plan next week.

Mr Poroshenko also said that he would discuss the possibility of a ceasefire at a meeting in Belarus on Monday of the Contact Group, which includes Ukraine, Russia, Belarus and the Organization for Security and Cooperation in Europe (OSCE).

The EU and the US have already imposed sanctions against dozens of senior Russian officials, separatist commanders and Russian firms accused of undermining Ukrainian sovereignty.

French President Francois Hollande said the Ukraine crisis was the biggest since the end of the Cold War.

He said: "What's happening in Ukraine is so serious that the European Council will be obliged to react by increasing the level of sanctions if things remain as they are."

However, the BBC's Chris Morris in Brussels says there are still divisions within the EU on how to deal with the Ukraine situation.

Finnish Prime Minister Alexander Stubb said the "jury is still out" on whether sanctions had worked, adding: "We need to find a ceasefire, a peace plan."

Federica Mogherini, named on Saturday as Catherine Ashton's successor, said there could be no military solution to the crisis and that while sanctions were being worked on, the diplomatic process would need to continue.

'Ordered to retreat'

Government forces have lost ground in recent fighting.

A Ukrainian military spokesman said on Saturday that Russian tanks had attacked the town of Novosvitlivka near Luhansk and "destroyed virtually every house".

Spokesman Andriy Lysenko said troops had been ordered to retreat from Novosvitlivka.

Troops are also reportedly trying to evacuate Ilovaisk in the Donetsk region. It has been surrounded by rebels.